The purpose of this project is to analyze the role of CD81, a component of the complement receptor 2 (CR2) complex, along with CD21, CD19 and Leu13, in signaling via the B cell antigen receptor (BCR). Co-ligation of the CR2 complex with the BCR amplifies signal transduction through the BCR. In an effort to understand the role of CD81 in B cell development and function, the investigator has generated mice with the CD81 gene disrupted. Preliminary analysis of these mice reveals grossly normal T and B cell development except for a large decrease in the number of B1 cells. CD81-/- mice expressed markedly decreased amounts of CD19 on their B cells, yet they had increased serum IgM and IgA, exhibited an exaggerated antibody response to the type II T-independent antigen TNP-Ficoll, and produced anti-DNA antibodies. These results suggest that B cells from CD81-deficient mice are hyperresponsive to signaling via the BCR. Dr. Tsitsikov proposes to investigate this hyperresponsiveness and to delineate residues in CD81 that are important for its function in modulating BCR signaling. The specific aims are: 1) to perform a detailed analysis of BCR signaling in CD81-deficient B cells, including examination of B cell proliferation, protein tyrosine phosphorylation, and calcium fluxes following crosslinking of membrane IgM, 2) to analyze the functional and physical interactions of the BCR and its co-receptors, CD19 and CD22 and phosphatases, and 3) to perform a mutational analysis of CD81 function in a murine B cell line and in transgenic mice expressing mutant forms of CD81. The proposed studies of the role of CD81 in BCR signaling may have important implications for the understanding and treatment of autoimmune diseases.